This invention relates to novel rubbery compositions, and in particular to tire treads wherein homopolymers of polybutadiene are the essential rubbery polymers.
Conventional automotive tire treads are based on vulcanizates of rubber blends filled with large amounts of oil and carbon black. As the rubber blends, there have been employed mixtures of copolymers of butadiene and styrene (SBR) with stereoregular polydiolefins (DR), preferably cis-1,4-polybutadiene prepared by emulsion or solution polymerization. The blending weight ratio, in this process of SBR : DR normally varies between 80 : 20 and 50 : 50 respectively. If the proportion of stereospecific polydiolefins is increased, the tread life is increased, but conversely, the tread skid resistance on wet roads is impaired. Consequently, from the standpoint of safety, manufacturers have been forced to accept only a limited usage of the more durable stereospecific polydiolefins. Aside from skid resistance, another problem associated with the exclusive use of the previously known homopolybutadienes, is the substantial difficulty in processing the formulations containing said homopolymers.
As a result of the above problems, it has been considered essential, heretofore, to employ the above-mentioned blends, and to tailor the composition of the blends to the respective requirements. However, the use of these blends, as compared for example, to a single polymer, involves more complex and expensive manufacturing operations. Furthermore, off-grade products and operational problems are encountered which often result from the fact that the blend components, which differ in their chemical constitution from one another, are very difficult to distribute homogeneously. Because of this blending problem and the relatively expensive manufacture of these blends, there has been a continuing need and significant interest in finding new polymers which would exhibit the same compromise in properties as the vulcanized blends, but which would be amenable to easier and more economical processing conditions.
Therefore, substantial research has already been conducted to provide improved polymeric systems for tire tread. Thus, it has been determined that elastomeric properties which are usable in certain respects are exhibited not only by homopolymers of diolefins having high contents of 1,4-cis-structure, particularly 1,4-cis-polybutadiene, but also by homopolymers having very high contents (above 50%) of monomer units disposed in the 1,2-position. Specifically, E. W. Duck and J. M. Lock (Jour. I.R.I. 2 (1968), p. 223) discovered that polybutadienes containing more than 50% of monomer units arranged in the 1,2-position do not exhibit any cold flow and have a higher building tack than other polymers and, furthermore, by and large have very useful rubbery properties. Unfortunately, however, these polybutadienes, having above 50% of monomer units disposed in the 1,2-position, exhibit an abrasion resistance which is clearly inferior to that of the standard SBR-types and especially to that of blends of SBR with cis-1,4-polybutadiene.